The problem of amplifying optical signals for long distance transmission was successfully addressed by the development of Erbium doped fiber amplifiers (EDFAs). An EDFA consists of a length of silica fiber with an active core doped with ionized atoms (Er3+) of the rare earth element Erbium. The fiber is pumped with a laser at a wavelength of 980 nm or 1480 nm. The doped, pumped fiber is optically coupled with the transmission fiber so that the input signal is combined with the pump signal in the doped fiber. An isolator is generally needed at the input and/or output to prevent reflections that would convert the amplifier into a laser. Early EDFAs provided 30 to 40 dB of gain in C-band extending between 1530 to 1565 nm with noise figures of less than 5 dB. Recently, EDFAs have been developed that provide similar performance in the L-band (1565 to 1625 nm). In addition, other fiber amplifiers using Nd, Pr, Tm and other active materials in their active core are used for various applications at various wavelengths.
The performance of fiber amplifiers depends on a number of parameters including pumping efficiency, level of population inversion of the ions in the active core, amplified spontaneous emission (ASE) competing with the useful amplified signal, cross-sections and refractive indices of the active core and of the cladding surrounding the active core. In many fiber amplifiers ASE is a major obstacle to effective amplification of the desired signal and thus ASE has to be suppressed. For example, in producing an EDFA for amplifying signals in the S-band the relatively high absorption losses and low gains over the S-band render the selection of fiber and fiber profile very difficult. The problems are so severe that the prior art teaches interposition of external filters between EDFA sections to produce an S-band EDFA.
For example, Ishikawa et al. disclose a method of fabricating an S-band EDFA by cascading five stages of silica-based EDFA and four ASE suppressing filters in Ishikawa et al., “Novel 1500 nm-Band EDFA with discrete Raman Amplifier”, ECOC-2001, Post Deadline Paper. In Ishikawa et al.'s experimental setup, the length of each EDA is 4.5 meters. The absorption of each suppressing filter at 1.53 μm is about 30 dB and the insertion losses of each suppressing filter at 1.48 μm and 0.98 μm are about 2 dB and 1 dB respectively. The pumping configuration is bi-directional, using a 0.98 μm wavelength to keep a high population inversion of more than D≧0.7 (D, relative inversion) The forward and backward pumping powers are the same and the total pumping power is 480 mW. Ishikawa et al. show a maximum gain of 25 dB at 1518.7 nm with 9 dB gain tilt.
In a similar vein, U.S. Pat. No. 5,260,823 to Payne et al. teaches an EDFA with shaped spectral gain using gain-shaping filters. The inventors take advantage of the fact that the EDFA is distributed to interpose a number of the gain-shaping filters along the length of the EDFA, rather than just placing one filter at the end of the fiber. Yet another example of an approach using a number of filters at discrete locations in a wide band optical amplifier is taught by Srivastava et al. in U.S. Pat. No. 6,049,417. In this approach the amplifier employs a split-band architecture where the optical signal is split into several independent sub-bands, which then pass in parallel through separate branches of the optical amplifier. The amplification performance of each branch is optimized for the sub-band which traverses it.
Unfortunately, Payne's, Ishikawa's and Srivastava's methods are relatively complicated and not cost-effective, as they require a number of filters. Specifically, in the case of Ishikawa five EDFAs, four ASE suppressing filters and high pump power are required. Also, each of the ASE suppressing filters used by either method introduces an additional insertion loss of 1-2 dB. The total additional insertion loss is thus about 4-8 dB.
It has been found that the geometric and index profiles of a fiber can significantly affect the fiber's transmission characteristics. Most waveguides and fibers are designed to prevent injected radiation from coupling out via mechanisms such as evanescent wave out-coupling (tunneling), scattering, bending losses and leaky-mode losses. A general study of these mechanisms can be found in the literature such as L. G. Cohen et al., “Radiating Leaky-Mode Losses in Single-Mode Lightguides with Depressed-Index Claddings”, IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982, pp. 1467-72. L. G. Cohen et al. teach that varying the cladding profile can improve various quality parameters of the guided modes while simultaneously maintaining low losses. Moreover, they observe that depressed-index claddings produce high losses to the fundamental mode at long wavelengths. Further, they determine that W-profile fibers with high index core, low index inner cladding and intermediate index outer cladding have a certain cutoff wavelength above which fundamental mode losses from the core escalate. These losses do not produce very high attenuation rates and, in fact, the authors study the guiding behavior of the fiber near this cutoff wavelength to suggest ways of reducing losses.
U.S. Pat. Nos. 5,892,615 and 6,118,575 teach the use of W-profile fibers similar to those described by L. G. Cohen, or QC fibers to suppress unwanted frequencies such as ASE and thus achieve higher output power in a cladding pumped fiber laser. Such fibers naturally leak radiation at long wavelengths, as discussed above, and are more sensitive to bending than other fibers. In fact, when bent the curvature spoils the W or QC fiber's ability to guide radiation by total internal reflection. The longer the wavelength, the deeper its evanescent field penetrates out of the core of the fiber, and the more likely the radiation at that wavelength will be lost from the core of the bent fiber. Hence, bending the fiber cuts off the unpreferred lower frequencies (longer wavelengths), such as the Raman scattered wavelengths, at rates of hundreds of dB per meter.
Unfortunately, the bending of profiled fibers is not a very controllable and reproducible manner of achieving well-defined cutoff losses. To achieve a particular curvature the fiber has to be bent, e.g., by winding it around a spool at just the right radius. Different fibers manufactured at different times exhibit variation in their refractive index profiles as well as core and cladding thicknesses. Therefore, the right radius of curvature for the fibers will differ from fiber to fiber. Hence, this approach to obtaining high attenuation rates is not practical in manufacturing.
In response to this problem more recent prior art teaches distributed suppression of ASE at wavelengths longer than a cutoff wavelength in fiber amplifiers such as EDFAs. This is achieved by engineering fiber parameters including the index profile and cross sections of the core and cladding layer including the use of a W-profile refractive index. The approach is discussed in more detail in U.S. patent application Ser. No. 10/095,303 filed on Mar. 8, 2002.
Although the teaching contained in the above application provides for effective distributed suppression of ASE in a fiber amplifier, the fiber cross-section enables the coupling of radiation at wavelengths below the cutoff wavelength between the core and the cladding. This effect, also known as cladding mode resonance, produces artifacts or cladding mode coupling losses in the short wavelength range of interest where the signal is to be amplified. For a general discussion of cladding mode coupling losses the reader is referred to Akira Tomita et al., “Mode Coupling Loss in Single-Mode Fibers with Depressed Inner Cladding”, Journal of Lightwave Technology, Vol. LT-1, No. 3, September 1983, pp. 449-452.
Cladding mode loss is a problem encountered in fiber Bragg gratings. One solution is to extend a photosensitive region in the core beyond the core to suppress cladding mode losses as taught in U.S. Pat. No. 6,351,588 to Bhatia et al. entitled “Fiber Bragg Grating with Cladding Mode Suppression”. U.S. Pat. No. 6,009,222 to Dong et al. also teaches to take advantage of a W-profile refractive index to confine the core mode and cladding modes thus reducing their overlap and coupling. Related alternatives to confining the core mode to suppress cladding mode losses are found in U.S. Pat. No. 5,852,690 to Haggans et al. and U.S. Pat. No. 6,005,999 to Singh et al.
Unfortunately, the approaches which are useful in suppressing cladding mode losses and avoiding cladding mode resonance in fiber Bragg gratings can not be applied to fiber amplifiers. That is because of fundamental differences in fabrication, construction and operating parameters between fiber Bragg gratings and fiber amplifiers with distributed suppression of ASE. Therefore, there is a need for fiber amplifiers having distributed suppression of ASE at wavelengths longer than a cutoff wavelength to be able to suppress cladding mode resonance or the coupling of radiation between the core and cladding at wavelengths shorter than the cutoff wavelength. It would be particularly useful to provide an EDFA having these capabilities where the wavelengths below the cutoff wavelength are contained in the S-band.